This invention relates to shingling of delicate conveyed sheet material, and more particularly to shingling of sheets of delicate corrugated paperboard or the like which are being conveyed at relatively high speeds. The invention may be applied to downstacking devices such as those disclosed in Marschke U.S. Pat. No. 4,200,276 and Thomas U.S. Pat. No. 4,598,901.
In U.S. Pat. No. 4,200,276, a continuous web of corrugated paperboard or the like is formed in an upstream processing device and is conveyed downstream where it is cut into sheets which are fed in line through an infeed nip to a vacuum conveyor section where the sheets are shingled. The shingled sheets are then fed through several downstream conveyor sections to a sheet stacker. U.S. Pat. No. 4,598,901 discloses a generally similar device, except that it includes two vacuum conveyor sections, one which pre-shingles the sheets and the other which re-shingles the pre-shingled sheets. The patents disclose numerous controls for variable speed motors and other apparatus.
The vacuum conveyor sections disclosed in the aforesaid patents include spaced upstream and downstream shafts, one of which is driven, with belts trained around the shafts and forming conveying means for the sheets. A vacuum box or plenum is disposed between the shafts and between the upper and lower belt flights. The plenum is connected to a source of negative pressure and is provided with opening means in its upper wall to apply a vacuum to sheets being conveyed through the vacuum conveyor section. Although the opening means is disclosed in the said patents as being disposed at the downstream end of the plenum, the opening means may often be positioned at the upstream plenum end in such devices, which are generally wellknown.
Heretofore, and in machines such as those disclosed in the aforementioned patents which feed sheets generally horizontally in succession through an infeed nip and hence downstream through a vacuum shingling section, the vacuum shingler has been disposed with its upstream end at a level below the nip discharge, thus providing a drop for the traveling sheets between the infeed nip output and the vacuum shingler input. The shingler has previously been inclined upwardly in a downstream direction to facilitate proper shingling of the sheets as they skimmed across the shingler and were ultimately slowed by the continuously applied vacuum.
The operation of such known shinglers has been generally satisfactory at moderate sheet speeds, such as 500-650 ft./min. However, faster sheet speeds such as 1000 ft./min. have become increasingly desirable. One problem occurring with faster speeds, that of scattering of the shingles, has been addressed and basically solved by the dual-shingler system of the aforesaid U.S. Pat. No. 4,598,901. However, another problem has been observed as sheet spreads have increased.
As the sheets have traversed the vacuum shingler, they have had a tendency to fold transversely or buckle across their width. This so-called "beam breaking" has not only damaged the sheets but has also caused jam ups in the machine at the vacuum conveyor section, sometimes resulting in undesirable down time to clear the jam.
The cause of the buckling problem is believed to have been determined by the present inventors. It is believed that the sheets entering the vacuum shingler are not as strong or rigid as the shingler was designed to handle. When the paperboard is formed in the upstream processing device, it is in a damp or wet state. Previously, and at slow operational speeds, the web feed continuously from the upstream processing device has time to substantially fully dry before being cut into sheets and shingled. The shingler can handle the dry sheets, which have low moisture content and structural integrity. However, with increased machine speeds, the paperboard doesn't fully reach the dry state before entering the shingler, and may even be soggy at that point. Thus, as it drops downwardly from the infeed nip output onto the inclined conveyor, it tends to "beam break" or crease transversely. Furthermore, as the forward portion of the damp sheet is pulled down by the vacuum of the inclined shingler, the sheet is subjected to further buckling forces.
It is an object of the invention to solve the problem of buckling of delicate sheets at the vacuum shingler, whether the sheets are weakened due to undesirably high moisture content, or possibly of a gauge and/or material which is normally weak.
In accordance with the various aspects of the invention, the vacuum shingler for a plurality of sheets traveling in succession through an upstream infeed nip is provided with a vacuum modulating control which applies sheet slow-down forces primarily to only the tail ends of the sheets. In one embodiment, the control provides a substantially "on-off" vacuum operation, with the vacuum "off" when the leading end portion of the sheet is adjacent the vacuum plenum opening means, and with the vacuum "on" when the tail end portion of the sheet is adjacent the opening means. In another embodiment, the control modulates the vacuum so that it is at a basically unshingling low or reduced level when the leading end portion of the sheet is adjacent the opening means, and is at a substantially increased working high level when the tail end portion of the sheet is adjacent the opening means so that basic shingling occurs.
In the present embodiment, the vacuum modulating control includes further control devices for sensing the conveyor speed as well as the position and length of the traveling sheets, with this information being processed to provide an output signal which controls the speed of a shaft with which a rotary valve is associated. The valve is disposed within the vacuum shingler plenum and modulates the flow of air through the plenum opening means in correlation with the determined information.
In addition, the vacuum shingler assembly is disposed so that its sheet input is substantially horizontally aligned with the discharge of the infeed nip. Furthermore, the assembly is disposed with its conveyor portions generally horizontal, rather than inclined, so that even the middle of a sheet is supported thereby as the sheet spans the conveyor.